Refine task_spawn with defense-in-depth security

[jserv]

This splits mo_task_spawn into two kernel-internal functions:

• mo_task_spawn_kernel(): Creates M-mode tasks (used by logger)
• mo_task_spawn_user(): Creates U-mode tasks (used by main, syscall)

The public mo_task_spawn() macro routes automatically based on CONFIG_PRIVILEGED build flag, maintaining backward compatibility.

Security model (three-layer defense-in-depth):

1. Compile-time: private/task.h guard rejects non-kernel builds
2. Runtime: in_syscall_context check rejects syscall-context calls
3. Hardware: read_csr(mstatus) traps immediately from U-mode

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Summary by cubic

Split task creation into kernel and user paths with defense-in-depth security, and simplified the app API to mo_task_spawn(entry, size) that auto-routes privileges. Renamed the syscall to sys_task_spawn and updated docs and examples.

• New Features

  • Added mo_task_spawn_kernel (M-mode) and mo_task_spawn_user (U-mode); mo_task_spawn macro routes based on CONFIG_PRIVILEGED.
  • Defense-in-depth: compile-time guard in private/task.h, runtime in_syscall_context check, hardware mstatus trap on U-mode.
  • Centralized creation logic via task_spawn_internal; logger and kernel bootstrap use explicit kernel/user APIs.
  • Renamed syscall to SYS_task_spawn with sys_task_spawn wrapper; documentation updated.

• Migration

  • Replace mo_task_spawn(fn, size, mode) with mo_task_spawn(fn, size).
  • Replace sys_tadd(...) with sys_task_spawn(...).
  • No app-side privilege mode needed; privileged builds create M-mode tasks automatically.

^{Written for commit f61d8ac. Summary will update on new commits.}

[github-actions]

Linmo CI Test Results

Overall Status: ✅ passed
Timestamp: 2026-01-28T07:00:10+00:00
Commit: 516f90c

Toolchain Results

Toolchain	Build	Crash Test	Functional
GNU	✅ passed	✅ passed	✅ passed
LLVM	✅ passed	⏭️ skipped	⏭️ skipped

Application Tests

App	GNU	LLVM
cond	✅ passed	⏭️ skipped
coop	✅ passed	⏭️ skipped
cpubench	✅ passed	⏭️ skipped
echo	✅ passed	⏭️ skipped
hello	✅ passed	⏭️ skipped
mqueues	✅ passed	⏭️ skipped
mutex	✅ passed	⏭️ skipped
pipes	✅ passed	⏭️ skipped
pipes_small	✅ passed	⏭️ skipped
pipes_struct	✅ passed	⏭️ skipped
prodcons	✅ passed	⏭️ skipped
progress	✅ passed	⏭️ skipped
rtsched	✅ passed	⏭️ skipped
semaphore	✅ passed	⏭️ skipped
suspend	✅ passed	⏭️ skipped
test64	✅ passed	⏭️ skipped
test_libc	✅ passed	⏭️ skipped
timer	✅ passed	⏭️ skipped
timer_kill	✅ passed	⏭️ skipped
umode	⚠️	⏭️ skipped

Functional Test Details

Test	GNU	LLVM
mutex:data_consistency	✅ passed	⏭️ skipped
mutex:fairness	✅ passed	⏭️ skipped
mutex:mutual_exclusion	✅ passed	⏭️ skipped
mutex:overall	✅ passed	⏭️ skipped
semaphore:overall	✅ passed	⏭️ skipped
umode:[exception]_illegal_instruction	✅ passed	⚠️
umode:pass:_sys_tid()_returned	✅ passed	⚠️
umode:pass:_sys_uptime()_returned	✅ passed	⚠️

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Report generated from test-summary.toml

[HeatCrab]

LGTM!